Microwave resonant system with tunable



April 26, 1960 J. A. RICH MICROWAVE RESONANT SYSTEM WITH TUNABLE SLOW WAVE STRUCTURE SECTION Filed Nov. 4, 1957 /n Ver-tor by J am United States Patent O MICROWAVE RESONANT SYSTEM WITH TUN- ABLE SLOW WAVE STRUCTURE SECTION Joseph A. Rich, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Application November 4, 1957, Serial No. 694,445 9 Claims. (Cl. S15-3.5)

My invention relates to improved microwave resonant systems, and more particularly to such systems having tunable slow wave structure sections and to tuning structures therefor.

A continual demand exists for electron tubes capable of performing at higher and higher frequencies with reasonable efficiency. The adverse effects of transit time in planar electron tubes, and of transit angle effects in klystrons, and corresponding effects in other types of tubes have led to the development of traveling wave tubes of the type utilizing a helical coil or similar slow wave structure that slows the axial velocity of an electromagnetic wave to slightly less than the velocity of some axially directed electrons. These tubes do not have some of the limitations of the prior tubes because their beam does not interact with fields from spaced electrodes, but rather with a simple succession of alternating electromagnetic fields along the length of the tube.

Although traveling wave tubes have many advantages over the prior tubes, they also have some limitations. For example, they have a very low efiiciency and their band width is so great that stabilization is difficult to achieve. Also, since these tubes are quite long, the magnetic structure required to produce a field along their axis is very massive and expensive. The band width can be narrower, the efficiency increased, and the construction simplified by the use of a resonant spiral coil along which a standing electromagnetic wave is developed.

In my copending application, Serial No. 491,007, Tunable Microwave Resonant System, filed February 28, 1955, now Patent No. 2,888,600, and assigned to the assignee of the present invention, I disclose a resonant helix section that is tuned through change of the resonator length of a coaxial resonator coupled to the helix section. However, tuning can also be achieved, and the requirement for an external resonator obviated, by change of the main helical coil parameters.

Accordingly, one object of my invention is to provide a. new and improved tunable resonant system.

Another object is to provide a new and improved tunable resonant system including a slow wave structure.

A further object of my invention is to provide a new and improved electron tube having a resonant helical coil that is tuned through change of the main helix parameters.

These and other objects are attained in one form of my invention by the provision of an electron tube along the axis of which a helical coil is mounted between two conducting walls that refiect electromagnetic waves along the coil. A plunger having a reflecting surface and mounted to be moved along the tube axis to control the distance over which the coil field interacts with the electrons in the beam passing through the coil permits the resonant frequency of the electron tube to be adjusted.

The novel features that I believe are characteristic of my invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to ICC the following description taken in connection with the accompanying drawing, in which:

Fig. l is a side elevational view of one embodiment of my invention,

Fig. 2 is a side elevational view of a portion of another embodiment, and

Fig. 3 is a side elevational view of a portion of still another embodiment.

In all of the figures corresponding elements have been designated by corresponding reference numerals to facilitate comparison. Now referrring specifically to Fig. 1, I have illustrated an electron tube comprising an enclosure 10 that includes a cylindrical wall 12 the ends of which are closed by a front wall 14 and a rear end wall 15, both having centrally located apertures. The principal function of wall 12 is to provide an evacuated envelope. However, if made conductive it also provides an electrostatic shield for structure within this enclosure. On the other hand, walls 14 and 15 must be conductive for reasons that will become apparent as the discussion develops. In front end wall 14 the aperture is covered by a screen 16 that while acting as an anode for attracting electrons is perforated to permit the passage of electrons therethrough. On the periphery of this wall a cylindrical extension 17 provides a connecting ring to other components of the electron tube that are described below. On the exterior of rear end wall 15 a collector electrode 19 surrounds the aperture to gather electrons passing therethrough. Cooling means although required for cooling collector electrode 19, which is heated by electron bombardment, are not illustrated because such means are well known in the art and their omission simplifies the drawing.

A slow wave structure, here shown to be a helical coil 20, is connected between front end wall 14 and rear end wall 15 which, due to their conductivity, cause this coil to be resonant. A plunger 21 of conductive material having an aperture 22 encircles coil 20 for providing a movable reflecting wall for the electromagnetic waves of the coil field. The clearance between coil 20 and plunger' 21 provided by aperture 22 is preferably just sufiicient to pass coil 20 and if too large severely reduces the reflecting action of the plunger. Physical support for plunger 21 is provided by peripheral fingers 23 that engage cylinder wall 12. If required, similar fingers can be utilized at the edge of aperture 22 for engaging coil 20.

The means for moving plunger 21 include a threaded rod 26, the threads of which engage threads in the plunger so that when this rod is rotated the plunger is moved forward or backward, depending upon the direction of rotation of the rod. Rod 26 is driven externally of enclosure 10 by a sylphon bellows system 28 that transmits mechanical motion while providing an air seal.

The output signal from enclosure 10 is extracted by means here shown to be a capacitor probe 30 coupled to the coil field. Alternatively, a short section of crosswound helical coil wound about a segment of coil 20 can be used. Probe 30, which is supported by a sealproviding insulator 32, extends to the center of a coaxial line 34 that transmits the output signals to a utilization circuit (not shown).

The electron gun assembly for the tube comprises a cathode and heater assembly 35 including a planar emitting surface 37. The cathode assembly is-supported within and is in insulating relationship with respect to a grid cylinder 39 that in turn is supported from a sleeve 41 forming a part of a planar grid terminal 43. A control electrode or grid 45 is supported on the upper end of cylinder 39 in closely spaced relationship with respect to cathode 37. The side wall of the tube is completed by cylindrical insulator 46 mounted within cylindrical extension 17 of enclosure 10.

The interaction region of the beam in coil 20 and the eld thereof extends from the front end Wall 14 to the nearest surface of plunger 21. Thus, through movement of this plunger the length of this region can be changed to vary the tuning or resonance frequency of the resonant system. The percentage of tuning obtained for a given percentage change in length of interaction region is approximately the same. As an illustration of the tuning of one practical embodiment, a traveling wave tube was constructed having a helical coil of 21/1 turns per inch, a mean diameter of 13/16" and a wire diameter of Ma". The diameter of the enclosure was approximately 4 times the diameter of the helical coil. By changing the length of the interaction region from 14 cm. to 6 cm. the resonant frequency was changed from 450 megacycles to 100() megacycles.

In applications where it is desired to use a simpler driving element than a Sylphon bellows system, an embodiment such as shown in Fig. 2 can be used in which an insulating cylindrical envelope such as a quartz tube 50 is inserted between coil 20 and plunger 21 to provide a vacuum enclosure only in the immediate vicinity of the coil. The region of enclosure 10 exterior to envelope 50 can then be at air pressure and thus a nonsealed drive, such as a rod 52, used for moving plunger 21.

A structure for using another helical coil parameter, the pitch of the coil, is illustrated in Fig. 3 where, at the collector electrode end of enclosure 10, three cylinders 60, 62 and 64 are coaxially mounted. The outer cylinder 60 is provided with threads 65 for engaging threads in an end cap 67. The other cylinders 62 and 64 support, respectively, the inner and outer sections of a cylindrical Sylphon bellows system 69, and also provide bearings 70 for a plurality of rods 71 extending from this bellows system. Rods 70, which bear against an annular ring 73 contacting helical coil 20, are driven by an annular ring 75 through a bearing 76 that is in contact with cap 67 so that as cap 67 is rotated inwardly the rods 71 press on annular ring 73 to compress coil 20. When this cap is rotated outwardly, rods 71 are forced by the spring action of coil in an outward direction against the annular ring 75 and thereby permit extension of the coil. Thus, through rotation of cap 67, coil 20 can be extended or compressed depending upon the rotation of the cap. It can be shown that a change in pitch produced by a 20% change in coil length produces approximately a 20% change in tuning.

It has been shown that by changing a helical coil parameter such as the length of the interaction region or the pitch of the coil, considerable tuning of the resonant helical coil can be obtained. Although the disclosed embodiments refer specically to single wire helical coils, all can be applied without modification to cross-wound helical coils and the embodiments of Figs. l and 2 can be applied without modification to bitilar helical coils. The only additional structure required in Fig. 3 for bilar heli cal coil applications is an annular-shaped insulator between the initial turns of the two helical coils at the collector electrode end of the structure so that these coils do not make contact when they are compressed. Also, a1- though the disclosed embodiments show only a traveling wave type of electron tube, they can be applied to slow wave structures in many other types of tubes, such as klystrons, for example.

Although I have described my invention with respect to certain embodiments it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit of my invention. I intend, therefore, by the appended claims to cover all such modcations and changes as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electronic device, a resonant system comprising an enclosure having reflecting end walls, a slow wave structure mounted between said walls, a plunger having a reecting surface mounted for movement in said enclosure between said walls, said plunger having an aperture therein for providing clearance between said slow Wave structure and said plunger such that said plunger extends around said slow wave structure, and means mounted external to said enclosure for providing axial movement of said plunger.

2. The resonant system as defined in claim l, and a cylinder of insulating material mounted within said enclosure to extend radially between the outer periphery of said slow wave structure and the edge of the aperture in said plunger.

3. In an electronic device, a resonant system comprising an enclosure having reecting end walls, a slow wave structure mounted between said end walls, and means for extending and compressing said slow wave structure to tune said resonant system.

4. An electronic tube comprising a vacuum enclosure having conducting front end and rear end walls with centrally located apertures therein, an electron gun assembly mounted to project a beam of electrons through the aperture in said front end wall along the axis of said enclosure and through the aperture in said rear end wall, a collector mounted to receive the electrons passing through the aperture in said rear end wall, a slow wave structure mounted between said front and rear end walls along the axis of said enclosure, a plunger having a conductive surface mounted to move axially of said enclosure with the conducting surface toward said front end wall, said plunger having a centrally-located aperture therein for providing clearance with said slow wave structure, and a Sylphon bellows system for moving said plunger axially of said cavity whereby tuning of said electron tube is obtained.

5. An electron tube comprising an enclosure having conducting front end walls and rear end walls with centrally located apertures therein, an electron gun assembly mounted to project a beam of electrons through the aperture in said front end wall along the axis of said enclosure and through the aperture in said rear end wall, a collector electrode mounted to receive the electrons passing through the aperture in said rear wall, a slow wave structure mounted between said front end and rear end walls along the axis of said enclosure, a cylindrical insulator mounted around said slow wave structure closely adjacent thereto, a plunger having a conductive surface, said plunger being mounted to move axially of said enclosure with the conducting surface toward said front end wall, said plunger having a centrally-located aperture therein for providing clearance with said cylindrical insulator, and means for moving said plunger axially of said enclosure whereby tuning of said electron tube is obtained.

6. An electron tube comprising an enclosure having conducting front and rear end walls with centrallylocated apertures therein, an electron assembly for projecting a beam of electrons through the aperture in said front end wall along the axis of said enclosure and through the aperture of said rear wall, a collector electrode for receiving the electrons passing through the aperture in said rear end wall, a helix-shaped conductor extending along the axis of said enclosure for forming a. slow wave structure, and means for compressing and extending said helix-shaped conductor for tuning said slow wave structure.

7. In an electron beam device, means for establishing an electron beam along the predetermined path, a resonant system comprising an elongated helical slow wave structure positioned to surround said predetermined path, two electromagnetic wave reflecting walls positioned in electrically coupled relationship with said slow wave structure for making resonant the portion of said slow wave structure between said walls and apertured for the passage of an electron directed along said predetermined path, and means movable along the length of said elongated helical slow wave structure for varying the electrical length of said elongated helical resonant system for tuning said slow wave structure.

8. In an electron beam device, means for establishing an electron beam along a predetermined path, a resonant system comprising first and second spaced electromagnetic wave reflecting walls, said second wall having an aperture therein, an elongated helical slow wave structure electrically connected to said first wall and extending through said aperture in said second wall whereby said first and second walls render resonant the portion of said an elongated helical slow wave structure extending therebetween, and means for varying the relative position of said elongated helical slow wave structure and said second reflecting wall in the direction of the length of said elongated helical slow wave structure to tune said resonant system.

9. In an electronic device, a resonant system compris ing rst and second spaced electromagnetic wave rellecting walls, said second wall having an aperture therein, a compressible slow wave structure with one end connected to said rst end wall, said slow wave structure extending through said aperture in said second wall whereby said rst and second walls render resonant the portion of said slow wave structure extending therebetween, and means for compressing and extending said slow wave structure for tuning said slow wave structure.

References Cited in the le of this patent UNITED STATES PATENTS 2,630,544 Tiley Mar. 3, 1953 2,825,841 Convert Mar. 4. 1958 2,878,412 Lally Mar. 17, 1959 FOREIGN PATENTS 1,081,836 France Iune 16, 1954 

